Bias voltage under fault condition (TOP258)
Hi,
I´m involved in the design of a four output power supply (12V/1,8A;18V/150mA;24V/100mA;33V/50mA) with a total output power of 28W. During normal working, bias voltage is 18V aprox. but when there is a short circuit and Top258 enters in fault condition mode, every restart attempt bias voltage grows up over 40V. I´ve disconected Vr2 zener diode of RDK142 and I´ve noticed that under same conditions bias voltage remains under 20V. I know that with a zener diode I can limit the bias voltage but I´d like to know what is the difference between both power supplies.
In order to improve cross regulation at maximun I´ve placed bias winding as the outer layer (P-S1-S2-S3-P-Bias) while in RDK142 the sequence is (P-Bias-S1-S2-P) and I suspect that this the cause but any comment or sugestion will be very appreciated.
Best regards
Comments
I´ll try it but the problem is that secondary cross regulation will be hurt. Another solution that it seems to works is to place 47k resistor in paralell with bias capacitor. Power consumption is below 8mW during normal working and discharge the capacitor during fault condition in less than 1s. Any comment?
Your solution can work, just be sure to examine all loading and line conditions, and if OVP works properly (if desired).
Placing the bias winding next to the secondary windins will not work secondary cross regulation IF the bias winding is NOT placed BETWEEN secondary windings.
Regards.
You are correct that in order to improve the tracking of the bias supply with the output, the bias winding should be placed next to the secondary winding.
The purpose of VR2 is not to limit the bias voltage, but to implement OVP. See page 8 of the report:
http://www.powerint.com/sites/default/files/PDFFiles/rdr142.pdf